1. Field of the Invention
The present invention relates to a technique of realizing skew detection of a sheet with a simple configuration, in a sheet carrying apparatus that carries a sheet.
2. Description of the Related Art
Conventionally, a technique of detecting a paper jam by using plural sensors arranged on a sheet carrying path in a sheet carrying apparatus that carries a sheet has been known. The plural sensors arranged as described above detect passage timing of the forward edge or rear edge of a sheet carried along the carrying path. When this passage timing has exceeded a preset allowable limit value, it is determined that sheet clogging (so-called a jam) has occurred.
In order to detect a skew (slanting) of a sheet, there is also a known technique of arraying plural sensors in a direction orthogonal to the carrying direction (see FIG. 9), then measuring the difference in passage timing between the sensors, and thereby determining the skew (for example, see JP-A-10-53355).
There is also a known technique of detecting the time for the forward edge of a sheet to pass between a sensor that detects passage of the sheet and an edge sensor that detects a lateral shift of the sheet, then comparing this time with the passage time in the same sensor section in a normal state (without a skew), and detecting the quantity of skew from the result of the comparison (JP-A-2005-350155).
However, in the technique described in JP-A-10-53355, since the plural sensors must be arrayed in a direction orthogonal to the carrying direction in order to detect the skew of a sheet, there is a problem that the number of sensors in the entire apparatus is increased.
Meanwhile, in the technique described in JP-A-2005-350155, since the edge sensor is used to detect the quantity of skew, there is a problem that the quantity of skew changes depending on the sheet size in the lateral direction (sheet width). Recently, the miniaturization of the apparatus causes reduction in the length of the carrying path and increase in the sheet carrying speed. Therefore, there may be inconvenience with the edge sensor that has limitations in its installation conditions, for example, a predetermined spacing to the registration roller is required in the carrying direction.
Moreover, in the technique described in JP-A-2005-350155, if the carrying time is changed by wear of a roller or the like, the carrying time of a sheet to be a detection target is influenced by the change in the carrying time due to the roller and the quantity of skew cannot be accurately determined.